The present invention relates to a high-transverse-curvature tire, having a curvature ratio not lower than 0.3, in particular for use in motor-vehicles, comprising:
a carcass structure of toric form having a high transverse curvature and provided with a central crown portion and two sidewalls ending in a couple of beads for anchoring onto a corresponding mounting rim;
a belt structure, circumferentially inextensible, coaxially extended around the carcass structure;
a tread band coaxially extended around the belt carcass and comprising a plurality of rubber blocks defined between a plurality of grooves extending along a direction substantially transverse to the running direction of the tire, the tread band being free from any groove extending along a direction substantially parallel to the running direction of the tire, said grooves comprising a bottom connected to opposite inlet and outlet sidewalls extending substantially perpendicularly to said bottom.
In the following description and in the appended claims, the expression: xe2x80x9csidewalls substantially perpendicular to the groove bottomxe2x80x9d is used to indicate walls that formxe2x80x94with respect to a plane perpendicular to the bottomxe2x80x94an angle varying from 0xc2x0 to 40xc2x0.
More particularly, the invention relates to a front tire for two-wheeled motor-vehicles, wherein the high transverse curvature is defined by particular values of the ratio between the height of the tread crown from the line passing through the tread axial ends or xe2x80x9ccamberxe2x80x9d of the tread band, said line being measured at the equatorial plane, on the one hand, and the distance between said tread band ends on the other hand. Said value, preferably not lower than 0.3, is anyhow higher than the value related to the corresponding rear tire, usually not lower than 0.15.
In the following description and in the appended claims, said ratio will be indicated by the term: xe2x80x9ccurvature ratioxe2x80x9d.
As is known, tires for two-wheeled vehicles have been manufactured for a long time with a carcass structure comprising a couple of plies of rubberized fabric reinforced with cords symmetrically inclined with respect to the tire equatorial plane, which structure is usually known as cross-plies carcass, and possibly a belt structure also comprising couples of strips of rubberized fabric provided with cords inclined with respect to the tire equatorial plane.
While such tire structure could ensure an extremely regular curve holding of the motor-vehicle, the use of this type of tires involved problems of comfort, stability, road holding of the vehicle and weariness of the driver, due to their excessive stiffness.
The structure of these tires, in fact, accumulatedxe2x80x94under the effect of an imposed deformationxe2x80x94elastic energy which was given back almost instantaneously upon termination of the stress, amplifying the unevennesses transmitted by the road surface layer, with ensuing stability loss of the vehicle.
In particular, during the straightaway running such excessive stiffness causes, at low speeds, high frequency (8-10 Hz) oscillations on the front tire (shimmy effect), causing driving to be precarious.
To try to obviate these problems, it has recently been proposed in co-pending patent application Ser. No. 08/857,305, filed on May 16, 1996 by the same assignee, the use of a radial-ply tire provided with a belt structure comprising circumferentially-oriented cords, preferably metal cords, also indicated by the term: zero-degree cords.
This type of tire has unquestionably improved the situation in terms of comfort and driving stability: in fact, the oscillations of the vehicle at straightaway high speeds have disappeared and in particularly the shimmy effect has been substantially eliminated.
However, even if improvements as to the comfort and reduction of the driver""s weariness have been achieved, the tires of the aforesaid type have not allowed, at least so far, neither to improve the wet grip of the vehicle nor to increase the wear resistance and, consequently, the kilometric yield of the tread band.
According to the present invention, the inventors have now realized that the problem of achieving such desired characteristics can be solved by combining a belt structure having zero-degree cords and a particular pattern of the tire tread, such as to provide a given ratio between the area occupied by the rubber blocks and the area occupied by the grooves (or solid/hollow area ratio).
Therefore, the present invention provides a tire of the type indicated hereinabove, which is characterized in that:
a) said belt structure comprises at least a radially external layer including a plurality of circumferential coils, axially arranged side by side, of a cord wound at a substantially zero angle with respect to the equatorial plane of the tire; and in that
b) the area occupied by said rubber blocks in a portion of tread band having a length equal to a pitch of the tread pattern and a width equal to the axial development of the tread band is between 70% and 90% of the total area of said portion.
According to the invention, the Applicant has found that by adopting a belt structure with zero-degree cords it is possible to obtain a higher degree of freedom in the design of the tread pattern, substantially overcoming the limit represented by the need of having a large solid area (i.e. a large area occupied by the rubber blocks) to ensure an adequate wear resistance of the tire.
According to the invention, in fact, it has surprisingly been found that by adopting a belt structure with zero-degree cords, it is possible to reduce the area of the rubber blocks, and therefore the solid/hollow area ratio, with respect to that of the tires of the prior art without affecting the wear resistance of the tire and achieving at the same time the following important advantages:
a) an improvement of tire capacity of draining off the water present under the tire ground-contacting area (aquaplaning);
b) an improvement of shimmy behavior of the tire in spite of the presence of a greater hollow area under the tire ground-contacting area;
c) maintenance of a high directional stability of the tire, whatever the use conditions, and especially while traveling over a curvilinear path;
d) an improvement of braking behavior of the vehicle with a reduction of the vehicle braking distance.
More particularly, according to the invention, such characteristics are achievable when, as indicated hereinabove, the area occupied by the rubber blocks formed in a portion of the tread band having a length equal to the pitch of the tread pattern and a width equal to the axial development of the tread band, is between 70% and 90% of the total area of said portion.
In the following description and in the appended claims, the term: axial development of the tread band, is used to indicate the extension in width of the latter as measured along the peripheral surface of the tire.
In the following description and in the appended claims, the term: pitch of the tread pattern, is used to indicate the length, measured along the circumferential development of the tread band, of a portion of the tread pattern which periodically repeats for a finite number xe2x80x9cnxe2x80x9d of times throughout the whole circumferential development of the tread band.
In the present case, therefore, the pitch of the tread pattern is equal to the distance between the starting points of two subsequent repetitive portions of the tread pattern, measured along the circumferential development of the tread band.
Preferably, the solid area is between 80% and 85%, and, still more preferably, it is equal to about 83% of the total area of said portion having a length equal to the pitch of the tread pattern and a width equal to the axial development of the tread band.
Advantageously, furthermore, the adoption of a belt structure with zero-degree cords in a radially external layer of the same allows to increase both the directional stability and the tire ground-contacting area, i.e. the area where the tire gets in touch with the ground, in any use condition of the tire.
Thanks to this feature, both the stresses due to sliding on the road and the stresses due to hysteresis dissipation in the rubber composition of the tread band are reduced, with an advantageous increase in wear resistance of the tire.
Preferably, the cord coils comprise high elongation cords made from high carbon content steel wires.
Alternatively, the cord coils may comprise aramidic textile cords.
Preferably, furthermore, the cord coils arranged at a substantially zero-degree angle with respect to the equatorial plane of the tire are distributed with variable density along the axial development of the belt structure.
According to this embodiment of the invention, the distribution density of the cord coils changes progressively along the belt structure, preferably progressively increasing from the equatorial plane towards the ends of the belt structure according to a predetermined relation.
In this way, it is advantageously possible to obtain a belt structure which is at the same time flexible in the middle, to absorb and damp the vibrations due to the ground roughness, and rigid at the sides, to develop high slip thrusts.
In accordance with the experiments of the Applicant, such relation may conveniently have the following expression:   Nx  =      K    ⁢                  R        2                    r        2              ⁢    No  
wherein:
No is the number of cord coils arranged in a central portion of unitary length located on either side the equatorial plane;
R is the distance between the center of said portion and the rotation axis of the tire;
r is the distance between the center of the unitary portion between the equatorial plane and the axial ends of said radially external layer and the rotation axis of the tire;
K is a parameter that takes into account the constituent material and the cord formation, as well as the amount of rubber around the cord and the weight of the radially-internal layer portion at said unitary portion, which is variable with variations in the material type and structural features of the belt strips along the crown profile that diverge from a reference value.
This parameter K may take a value substantially close to 1 if the cords have the same formation and all the connected materials are the same throughout the layers, or different values according to variations in the materials and formation of the reinforcing elements along the peripheral extension of the belt structure.
A distribution of the cords in accordance with such relation ensures both the uniformity of the stress acting on the belt structure during use of the tire, as a consequence of the centrifugal force applied, and the necessary differentiated stiffness along the axial direction.
Clearly, those skilled in the art may find other relations which, according to the aforementioned design variables, would allow to achieve at the same time a differentiated stiffness along the axial direction and a stress uniformity in the belt structure of the running tire, by varying in a controlled manner the density of the above cords.
Preferably, the winding density of the cords at zero-degree in the area located on either side of the equatorial plane, where the maximum thinning out takes place, is not greater than 8 and more preferably is between 3 and 6 cords/cm.
The axial width of said zone varies preferably from 10% to 30% of the axial development of the belt.
Preferably, the quantity of cords in said central area is equal to a value between 60% and 80% of the quantity of cords near the tire shoulders, where the density of said cords is preferably not greater than 10 and more preferably included between 6 and 8 cords/cm.
The cord coils of the aforesaid radially external layer are wound on a reinforcing layer in a radially internal position, which, in a preferred embodiment, is essentially consisting of a sheet of elastomeric material placed between said cord coils and the carcass ply, possibly filled with binding agents dispersed in said material.
Preferably, said binding agents comprise reinforcing fibrous fillers of a material selected from the group comprising: textile, metal and glass fibers or short fibrillated aramid fibers, randomly arranged or oriented according to a preferred direction, inclined with respect to said equatorial plane.
Preferably, said reinforcing fibrous fillers are homogeneously distributed in said elastomeric matrix with a density per volume unit of from 0.5% to 5% of the total volume.
Still more preferably, the reinforcing fibrous fillers are short fibers formed with fibrils of aramid homogeneously distributed in the sheet of elastomeric material in a quantity of from 1 and 10 parts by weight per 100 parts by weight of elastomer (phr).
According to a further embodiment, the radially internal layer may comprise two strips axially arranged side by side, provided with reinforcing elements oriented according to inclined directions in each strip and opposite to one another in the two strips with respect to the equatorial plane of the tire.
Alternatively, the radially internal layer may comprise two radially superposed strips provided with reinforcing elements parallel to one another in each strip and crossed with the elements of the adjacent strip, symmetrically inclined with respect to the equatorial plane of the tire; in this case, the reinforcing elements in one of said strips can be of a different material from that of the reinforcing elements of the radially adjacent strip.
Conveniently, the reinforcing elements of said radially internal layer are selected from the group comprising textile cords and metal cords.
Advantageously, and in any embodiment, said radially internal layer may be interrupted in correspondence of the equatorial plane for a portion of width preferably ranging from 10% to 30% of the axial development of said belt.
In a preferred embodiment of the present invention, the grooves transversely extend along the tread band according to a curvilinear path substantially parallel to the so-called wear waves (also known by the term xe2x80x9cSchlamack wavesxe2x80x9d, after the name of the researcher who has theorized the phenomenon) of the tread band.
Such a conformation of the grooves, in fact, advantageously allows to reduce the wear of the tread band and contributes to reduce the tire noise during running of the vehicle.
Preferably, just to better follow the aforesaid wear waves, the grooves have at least a curvature center positioned upstream of the same at the opposite side zones of the tread band, external to said equatorial zone.
In such side zones, the grooves preferably have a curvature radius of from 120 to 180 mm.
Preferably, the tire of the present invention comprises at least a groove substantially extending throughout the whole axial development of the tread band according to a substantially double-inflection curvilinear path, including opposite lateral portions having their respective curvature centers positioned upstream of the groove and on opposite sides with respect to said equatorial plane of the tire.
Also in this case, the opposite lateral portions of such a groove preferably have a curvature radius of from 120 to 180 mm.
Preferably, furthermore, at least one of the lateral portions of such double-inflection path groove transversely extends along the tread band through substantially the whole width of the equatorial zone of the tire and the whole width of one of said side zones of the tread band.
Preferably, said opposite lateral portions of the groove having a double-inflection path are connected through an intermediate portion, having a curvature center positioned downstream of the groove, extending externally to the equatorial zone and in at least a part of one of the side zones of the tire tread.
Preferably, such intermediate connecting portion has a curvature radius of from 20 to 40 mm.
According to this embodiment of the invention, the grooves having a double-inflection path act as connecting elements between grooves formed in opposite parts of the tread band and shaped according to the wear waves.
Advantageously, the double-inflection grooves, not only optimize the wear of the tread band in terms of uniformity and reduction of wear velocity, but also contribute to a still more effective evacuation of the water present under the tire ground-contacting area.
In this way, the draining capacity of the tire improves to such an extent that, when it is mounted on the front wheel of a motor-vehicle, the rear tire travels on a xe2x80x9ctrackxe2x80x9d substantially free from water.
According to a further embodiment of the present invention, particularly preferred in case of high performance tires, in an equatorial zone of the tire the outlet wall of the grooves is inclined with respect to their bottom towards a direction opposite to the running direction of the tire and forms with respect to a plane (xcfx80) tangent to said bottom an angle (xcex1xe2x80x2) of from 100xc2x0 to 130xc2x0.
In the following description and in the appended claims, the terms: xe2x80x9cinletxe2x80x9d and xe2x80x9coutletxe2x80x9d are used to indicatexe2x80x94with reference to the structural features of the groovesxe2x80x94the parts of the grooves which are stressed first or get in touch first with the ground during tire rolling and, respectively, the parts of the grooves that are stressed after a predetermined angular rotation of the wheel.
In the same way, in the following description and in the appended claims, the terms: xe2x80x9cupwardsxe2x80x9d and xe2x80x9cdownwardsxe2x80x9d are used to indicatexe2x80x94with reference to the position of the groovesxe2x80x94those parts of the tread band, for instance the rubber blocks, that are stressed or get in touch with the ground during the tire rolling before and, respectively, after said grooves.
In the following description and in the appended claims, furthermore, all the angular values measured, starting from a plane (xcfx80) tangent to the groove bottom will be measured in counterclockwise direction.
According to the invention, when the value of angle xcex1xe2x80x2 formed by the inlet wall of the grooves is within the range of values mentioned hereinabove, it has been noticed a rigidity increase of the rubber blocks positioned downwards of the grooves in the very zone of the tread band subject to greater stresses during running of the vehicle and in particular during braking, with a substantial reduction of wear phenomena and a simultaneous achievement of a more uniform wear of the tread band.
Thanks to this conformation of the grooves outlet walls, the following additional advantages have also been achieved:
a) possibility of reducing the weight of the tire and, along therewith, of reducing both the disturbing effects on vehicle trim caused by impacts or ground roughness and the braking distance because of the lower inertia of the tire;
b) an increase in braking capacity of the tire, which allows a further reduction of the braking distance of the vehicle;
c) a greater wear uniformity of the tire tread, with an ensuing advantageous increase in road holding of the same;
d) a lower rolling resistance of the tire, with an ensuing consumption reduction.
Preferably, the angle xcex1xe2x80x2 is between 110xc2x0 and 120xc2x0 and still more preferably, it is equal to about 115xc2x0: in fact, an optimum rigidity of the rubber blocks positioned downstream of the grooves has been found within such range of values, while above 130xc2x0 the tire has shown an undesired and progressive loss of grip, a wear increase and rolling unevenness.
Preferably, the equatorial zone of the tread band interested by the desired inclination of the outlet walls of the grooves extends on either side of the equatorial plane of the tire for a portion having a width of from 10% to 35% of the axial development of said tread band.
Still more preferably, such equatorial zone extends on either side of the equatorial plane of the tire for a portion having a width of from 25% to 30% of the axial development of the tread band.
Preferably, the inclination of the outlet walls of the grooves, i.e. the value of angle (xcex1xe2x80x2), is substantially constant within the equatorial zone of the tread band, as specified hereinabove.
It has in fact been found that such feature contributes to achieve, in the very tread zone more stressed during braking, the rigidity of the rubber blocks necessary to obtain a substantial reduction in the aforementioned phenomena of wearing irregularity, accompanied by a quick degradation of the outlet edges of the grooves and loss of braking capacity of the tire.
At the opposite side portions of the tread external to said equatorial zone, on the contrary, the value of angle (xcex1xe2x80x2) linearly reduces as one moves away from the equatorial plane (Xxe2x80x94X), and according to the chord of the tire, down to a minimum value of from 90xc2x0 to 100xc2x0, which value is reached near opposite end portions of the tread band.
In other words, the inclination of the outlet walls of the grooves progressively increases with respect to plane (xcfx80) tangent to their bottom, until it is reachedxe2x80x94only and solely in correspondence of opposite end portions of the tread bandxe2x80x94the configuration of xe2x80x9csubstantial perpendicularityxe2x80x9d shown by the grooves formed on the tires of the prior art throughout the whole axial development of the tread band.
Since the length of the grooves, according to the special tread pattern one wants to realize, may also be shorter than the whole axial development of the tread band, it ensues that the inclination of the outlet wall of the grooves takes a predetermined value, in accordance with the variation rule defined hereinabove, depending on its position (equatorial zone rather than side zones) on said tread band.
This means that the aforementioned variation in the inclination of the grooves inlet walls from 130xc2x0 to 90xc2x0 will take place only for those grooves having such a length as to span along the whole axial development of the tread band, while for those grooves positioned only in the side zones of the tread band and having such a length as to be outside of the equatorial zone, the variation of the values of xcex1xe2x80x2, inversely proportional to the inclination of the outlet walls with respect to plane xcfx80, may be limited to a range from, for instance, a maximum value of 115xc2x0 (minimum inclination of the wall) to a minimum value of 95xc2x0 (maximum inclination of the wall) as one moves away from the equatorial plane of the tire.
According to a preferred feature of the invention, furthermore, in the equatorial zone of the tread band, the inlet wall of the grooves formed in the tread band is inclined with respect to their bottom towards the rolling direction of the tire and forms, with respect to a plane (xcfx80) tangent to said bottom, an angle (xcex1) of from 80xc2x0 to 90xc2x0.
In other words, the inlet walls of the grooves show in the equatorial zone of the tread band a configuration of xe2x80x9csubstantial perpendicularityxe2x80x9d which imparts to the rubber block upstream thereof the mobility necessary to ensure an adequate road holding.
In the same way as has been illustrated hereinabove with reference to angle xcex1xe2x80x2 formed by the outlet wall of the grooves, angle xcex1 as well is preferably substantially constant throughout the whole equatorial zone of the tread band.
In accordance with a further preferred feature of the invention, the angle xcex1 formed by the inlet wall of the grooves linearly decreases as one moves away from the equatorial plane (Xxe2x80x94X), and according to the chord of the tire in the opposite side zones of the tread band external to said equatorial zone, down to a minimum value of from 50xc2x0 to 80xc2x0, reached at opposite end portions of the tread band.
Preferably, said angle xcex1 is between 60xc2x0 and 70xc2x0 and, still more preferably, it is equal to about 65xc2x0.
In other words, the inlet walls of the grooves progressively reduce their inclination with respect to the plane (xcfx80) tangent to their bottom and towards the tire rolling direction, until they reach, at opposite side zones of the tread band, a configuration xe2x80x9csymmetricalxe2x80x9d to that shown by the grooves of the equatorial zone of the tread band.
In this way, an optimum rigidity of the rubber blocks positioned upstream of the grooves, whichxe2x80x94coupled with the mobility of the downstream positioned rubber blocksxe2x80x94causes both an adequate directional stability and an adequate road holding of the tire, has been achieved in the side zones of the tread band.
In a further embodiment of the invention, it has been found that, by making the carcass beads of aramidic fiber, a tire structure is obtained that has an improved flexibility of the beads which makes the operation of mounting the tire on the rim easier.
Further characteristics and advantages will be more readily apparent by the following description of a preferred embodiment according to the invention, solely provided by way of non limiting indication, reference being made to the attached drawings.